target/riscv/kvm.c: Fix the hart bit setting of AIA

target/riscv/kvm.c: Fix the hart bit setting of AIA

In AIA spec, each hart (or each hart within a group) has a unique hart
number to locate the memory pages of interrupt files in the address
space. The number of bits required to represent any hart number is equal
to ceil(log2(hmax + 1)), where hmax is the largest hart number among
groups.

However, if the largest hart number among groups is a power of 2, QEMU
will pass an inaccurate hart-index-bit setting to Linux. For example, when
the guest OS has 4 harts, only ceil(log2(3 + 1)) = 2 bits are sufficient
to represent 4 harts, but we passes 3 to Linux. The code needs to be
updated to ensure accurate hart-index-bit settings.

Additionally, a Linux patch[1] is necessary to correctly recover the hart
index when the guest OS has only 1 hart, where the hart-index-bit is 0.

[1] https://lore.kernel.org/lkml/20240415064905.25184-1-yongxuan.wang@sifive.com/t/

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Message-ID: <20240515091129.28116-1-yongxuan.wang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>